A LAND SNAIL 97 



the air is drawn into the mantle cavity through the respiratory 

 pore; this is accompHshed by the alternate enlarging and con- 

 tracting of the cavity by means of the muscular body wall which 

 constitutes its floor. Notice the longitudinal and the transverse 

 muscles in this floor. The blood circulating in the lung is oxy- 

 genated and passes into the heart through the pulmonary vein 

 as arterial blood. It is forced by the heart through the aorta, and 

 thence through arteries to all parts of the body, whence it returns 

 through blood lacunae to the lung. 



The Excretory System. The large kidney has already been seen. 

 It is a sac the glandular projections of the walls of which almost 

 fill its lumen. As is the case with pelecypods, the kidney commu- 

 nicates with the pericardial space through a fine canal and also 

 with the mantle cavity by means of a ureter. The pericardial 

 canal is opposite the ventricle and cannot be seen easily. The 

 ureter may be easily traced. It is a wide canal which leaves the 

 kidney at its forward end near the place where the pulmonary 

 vein approaches the kidney ; it first runs along the inner side of 

 the kidney to its hinder end ; here it doubles on itself and passes 

 forward to the inner edge of the mantle, where it runs beside the 

 rectum to a point near the respiratory pore and opens into the 

 mantle cavity. 



It will be noticed that the heart and the kidneys are both asym- 

 metrical organs. The heart has but one auricle ; it will be remem- 

 bered that in the pelecypod the auricles are paired organs ; one 

 of the pair must thus be wanting in the snail. There is also only 

 one kidney and one ureter, instead of a pair of each, as in the 

 pelecypod. It is the left member of the pair in each case which is 

 wanting. 



Exercise 4. Draw a view of the inner surface of the mantle on a scale 

 of 3, showing the organs mentioned above ; label all. 



The Digestive System. Pass a bristle through the anus into the 

 rectum in order to mark it. With two strong pins firmly fasten 

 the extreme forward end of the animal's foot and also its hinder 

 end to the wax of the dissecting pan. With sharp, fine scissors cut 

 through the floor of the mantle cavity and the collar in the median 



